Otitis: Inside look at pathogenesis, treatment and prevention

Otitis externa is an inflammatory condition with or without concurrent infection and is the most common dermatological disease in a busy veterinary practice. Therefore, it is the most common claim forwarded to veterinary insurance companies.

For the most part, the majority of cases are simple, and treatment clears the disease in a few days. However, many cases are recurrent and the management of those cases is frustrating and often veterinarians seek the aid of a specialist. This paper will focus on the more complicated cases and will clarify and discuss the pathogenesis, treatment and preventive measures. I will not discuss the uncommon causes of otitis such as parasites, foreign bodies and neoplasia within the context of this article. The emphasis of this article will be on the dog with severe, complicated or recurrent bacterial otitis with a focus on the pathology and treatment.

Anatomy-normal

The canine ear varies in the size, shape and conformation from breed to breed. The external ear is comprised of the pinnae that can be large and pendulous or erect. It is speculated that a pendulous pinna predispose to otitis externa. However, even erect pinnal breeds such as the German Shepherd are a common breed affected by otitis external The ear has some hair (some breeds have excessive hair), modified apocrine sweat glands (cerumen glands) and sebaceous glands. A small amount of cerumen should be present in the opening to canals as well as on the surface of the canals, leading down to the tympanum membrane. Some dogs have small or stenotic canals (such as Shar Peis) as a breed-related problem. The canals are divided into vertical leading into the horizontal canal which ends at the tympanum membrane. The tympanum membrane separates the external ear canal from the middle ear and leads ventrally into the bulla. The tympanum membrane should be a bit opaque, not completely cloudy and should be relatively smooth.

The microbiology of the ear canal is very similar to the hair and skin in dogs. In my recent article concerning pyoderma, the normal bacterial flora was elucidated. Staphylococcus, E. coli, Corynebacteria, and even transient bacterial such as Proteus, Enterococcus and Pseudomonas can be found. Malassezia, as a yeast organism, is also found normally on the pinna and canal of dogs.

Predisposing causes

It is speculated that predisposing factors are not the primary cause of otitis externa/media but they either trigger or increase the risk of its development. In my practice, these predisposing causes are not common, but in this paper they are definitely worth a short discussion.

Conformation is the most common predisposing cause and includes the large and pendulous pinnal seen in certain breeds such as the Cocker Spaniel or Springer Spaniel. These breeds are also predisposed to other causes of otitis, so careful history and physical examination must be performed before one blames anatomy as the sole cause of otitis.

Excessive hair in the ear canal (or on the medial aspect of the pinnae) has also been implicated as a predisposing cause to otitis. Breeds such as Poodles and certain Asian breeds are considered the bulk of the breeds and most definitely benefit from routine plucking of hair from the vertical canals. I am very careful when I perform hair removal, because certain cases seem to worsen after this procedure. Moreover, some specialists seem to believe removal of hair can worsen and not benefit these patients. As I mentioned earlier, I uncommonly observe these cases, but when I do, I am very careful in selecting patients for hair removal.

Lastly, the one predisposing factor that can complicate and trigger otitis is swimming. Swimming has definitely been linked to otitis in human patients, and it seems that certain dogs are also affected with a similar problem. The real dilemma that I have is the most common breeds that swim are also the most common breeds affected by allergy: the most common primary cause of otitis.

There are numerous primary causes of otitis in the dog including parasites such as Otodectes, Demodex and pinnal parasites such as Sarcoptes, seborrhea, autoimmune diseases, non-yeast fungal infections and glandular disorders. We will focus our attention on the most common primary cause of otitis: atopic dermatitis. Because of its high incidence, atopic dermatitis is more frequently associated with otitis externa than any other disease.

It has been reported that 3 percent to 10 percent of atopic dermatitis cases present with otitis externa and less so with unilateral otitis externa. Generally speaking, if you examine closely the medial aspect of the pinnae of early cases of otitis externa with atopic dermatitis, you will see subtle erythema on central and medial aspect (concave surface just where the pinnae might "fold" in half) of the pinnae. This inflammation is the early stages of the disease and can lead to further increase in inflammation, particularly in the canal. Prolonged inflammation can thus lead to an increase in glandular secretions and eventually microbial overgrowth. It seems that pinnal inflammation is much easier to treat than canal inflammation. It also has been reported that 20 percent of food-allergy cases present soley with bilateral or unilateral otitis. Furthmore, more than 80 percent of confirmed food-allergy cases have at least otitis externa present. I have not found this to be true. I uncommonly confirm food allergy in the dog.

Perpetuating causes

Perpetuating causes include primarily microbial colonization or overgrowth (mainly bacteria and Malassezia). As mentioned earlier, progressive changes on the pinnae and in the ear canal can also perpetuate the ear disease. Finally, extension into the middle ear, termed otitis media can become a persistent nidus for inflammation and infection, and can also perpetuate otitis.

The ear canal is simply an extension of the skin, and can be similarly infected and affected by microbial infections, cutaneous pathologic changes such as glandular (modified apocrine sweat glands called ceruminous glands) and epidermal hyperplasia (lichenification) as well as edema, fibrosis and even calcification.

Chronic inflammation is the leading cause of these pathologic changes. It has been reported that breeds predisposed to otitis have many more modified apocrine glands in the ear canal compared to normal dogs. Chronic inflammation that is progressive leads to hypertrophy or hyperplasia of the epidermis, hyperkeratosis, follicular epithelium, glands, and results in microbial overgrowth, erosions, ulcerations, pain, fibrosis and, in some cases, calcification. Not all cases progress this way, many dogs never reach the calcification stage, but many dogs (at least histologically) are affected by fibrosis. Another important fact is that most cases of otitis media have progressed from otitis externa. Fibrosis results in narrowing of the canal, which is a common predisposing cause. The thickening or hyperplasia of the skin can also impede normal circulation, increase moisture and interfere with the otoscopic examination and proper cleaning. The increased epithelial secretions and debris can favor microbial overgrowth. Finally, a combination of microbial byproducts, glandular and epithelial debris and inflammatory mediators released from the primary and predisposing causes can further contribute to the pathology.

Bacterial otitis externa/media

Apparently, bacteria are considered to be an uncommon cause of primary otitis externa. They generally are the result of chronic allergy. Bacterial isolates from the ear canal include Staphyloccus, E. coli, Proteus, Enterococcus, Klebsiella and, uncommonly, Pseudomonas. Obviously, identification of the primary cause is most important. Identification of bacteria is simply performed with cytology. I prefer one that has a counter stain such as the Diff-Quick brand but Wright's stain will serve you well. I have not found Gram stain to be of benefit. Generally, cocci organisms in the ear canal are gram positive and rod-form bacteria are gram negative. Therefore, morphology of the bacterium is most important when examining cytology specimens. Gram-negative organisms can be present with otitis externa, but generally they are associated with otitis media. Most cases of otitis media have a ruptured tympanic membrane.

My criteria for performing a culture includes cases that are chronic or recurrent, patients that have a ruptured tympanic membrane, and cases that demonstrate rod form bacteria visualized with cytology.

Some of the most frustrating cases that I must deal with include cases of chronic pseudomonal otitis media. I believe these cases are the most frustrating cases to deal with due to a number of factors. The most difficult factor associated with pseudomonal otitis media is the significant resistance to antibiotics that this organism demonstrates. This genus of bacteria is routinely resistant to the penicillins (including cephalexin) and can be resistant to the fluoroquinolones and aminoglycosides. Another factor that leads to frustration includes the pain and inflammation associated with severe cases of pseudomonal infections. This pain can be a challenge to manage.

Before we discuss treatment options for bacterial otitis, I will mention briefly a few facts that can be helpful for treatment and management. When one obtains a culture and sensitivity, those values are based upon minimum inhibitory concentration on the serum or plasma that is needed to achieve a kill. Many of the antibiotics are also available in topical form and can be instilled into the ear canal directly on the bacteria, thus achieving a much higher concentration of the antibiotic in the canal. This is important because many antibiotics are more effective at higher concentrations.

Also, when microbiologists and microbiology technicians select a few colonies (after culture is complete) for sensitivity studies, the information may not reflect the resistance for the remaining colonies. That is why repeated cultures and sensitivities on the same ear canal and organism can lead to different results. I have found this phenomenon to be most frustrating.

Treating bacterial otitis externa

Generally, most cases are readily cleared with appropriate topical treatments and systemic antibiotics are not helpful. Neomycin, gentamycin and enrofloxacin containing otic preparations are the most commonly available products for treatment.

I generally recommend topical treatments with corticosteroids if inflammation is moderate and advise systemic corticosteroids if inflammation is severe. Most cases are cleared in two to three weeks and may require weekly or twice-weekly ear washes/rinse to be used as prevention. Products that contain acetic acid, boric acid, chlorhexidine with ceruminolytics or surfactants are most helpful. Clearly, identifying the primary cause is important and can include food trials and intradermal skin testing. I have not found antihistamines to benefit otitis externa.

Treatment of bacterial otitis media

As mentioned earlier, culture and sensitivity and cytological examination is very important in the proper management of otitis. After cytology is performed I advise a culture if appropriate and tend to be very aggressive with therapy. The bacterial cultures may need to be repeated several times during therapy. Initially, I advise sedation if there is a fair amount of discharge in the ear canal, pain with otoscopic examination, or if I cannot visualize the tympanic membrane readily. Routine flushes can be used to irrigate the canal during sedation and include saline or non-alcohol and surfactant containing otic preparations. I have not found an increase incident of ototoxicity with products that have a reputation for causing such damage instill into an ear canal with a ruptured ear drum.

If severe pain and/or hyperplasia is present (particularly in the Cocker Spaniel), I routinely prescribe anti-inflammatory dosages of prednisone. Recently, oral cyclo-sporine has proved beneficial in the treatment of severe and hyperplastic otitis externa. I select antibiotics based upon results of cytology and culture and sensitivity. Routinely, I use topical antibiotics as well as systemic antibiotics pending results of culture. Typically, I select aminoglycosides or fluoroquinolones for topical therapy.

For systemic therapy, I prefer oral Clavamox and sulfa-type antibiotics for non-pseudomonal infections and oral fluoroquinolones for pseudomonal infections. Outlined below are some recipes for topical therapies that will aid in the treatment of resistant pseudomonal infections.

I expect most cases of severe pseudomonal otitis media to be treated for four to six weeks with topical antibiotics (eliminate the corticosteroid-containing topical preparations in three to four weeks), eight to 12 weeks with systemic antibiotics. Generally, orally administered corticosteroids are ceased in two to three weeks.

Malassezia otitis usually is not complicated to treat, however I have seen several cases of severe, chronic, somewhat refractory cases of yeast otitis in the dog.

Cytology readily demonstrates the typical yeast organism, but on occasion, few or rare Malassezia organisms are found. I believe these latter cases are a variant of the routine yeast otitis case and should be treated similarly. Generally, I advise topical clotrimazole/betamethazone containing products or products that contain miconazole.

Most of these otic preparations are useful in treating the pruritus and pain and contain corticosteroids. In severe cases I recommend oral corticosteroids as I do in cases of severe bacterial otitis. Systemic ketaconazole can also be useful in severe or recurrent cases.

For maintenance, I prefer to search for the primary cause, reduce swimming and prescribe acetic/boric ear washes as a preventive treatment for long-term management.

Many cases of otitis externa (and even media) can be routinely treated in general practice. The cases that should be referred to a dermatologist (hopefully not a surgeon) are the ones that are recurrent in nature, resistant to treatment or need allergy testing.

Dr. Vitale received his veterinary degree from Mississippi State University, College of Veterinary Medicine. He completed a residency in veterinary dermatology at the University of California, Davis and is a diplomate of the American College of Veterinary Dermatology. He is a clinical instructor/lecturer at UC-Davis and a staff dermatologist at East Bay Veterinary Specialists (formerly Encina Veterinary Hospital), Bay Area Veterinary Specialists and San Francisco Veterinary Specialists.